CNC machining process principles

CNC machining process flow

Machining process planning refers to the entire process and cannot be judged by the nature of a particular process or the machining of a particular surface. For example, some positioning datums need to be machined accurately in the semi-finishing stage or even in the roughing stage. Sometimes, in order to avoid dimensional chain conversions, semi-finishing of certain minor surfaces can also be arranged in the finishing stage.

When the processing method and processing stage of the part surface are determined, the processing of each surface in the same processing stage can be combined into a number of work steps.

  1. Processing process division method

In the CNC machine tool processing parts, generally according to the principle of process concentration division process, divided into the following methods.

(1) according to the use of tools divided

To the same tool to complete the process as a process, this division method is suitable for the workpiece to be machined surface more situations. Machining centres often use this method.

(2) Divided by the number of times the workpiece is mounted

The process that can be completed with one clamping of the part is used as a process. This method is suitable for parts with little machining content, and the entire machining content is completed in one clamping under the premise of ensuring the machining quality of the part.

(3) According to the roughing and finishing division

The part of the process completed in rough machining as a process, the part of the process completed in finishing as another process. This division method is suitable for parts with strength and hardness requirements, the need for heat treatment or parts with high accuracy requirements, the need to effectively remove internal stress, as well as parts after processing deformation is large, the need to be divided by rough and finish machining stages of parts processing.

(4) Divided by processing parts to complete the same part of the surface process as a process.

For the processing surface and more complex parts, should be reasonable arrangements for CNC machining, heat treatment and auxiliary processes in order, and solve the problem of the interface between processes. 2. the principles of processing process division The part is composed of multiple surfaces, these surfaces have their own accuracy requirements, each surface also has the corresponding accuracy requirements between. In order to achieve the design accuracy requirements of the parts, processing sequence arrangement should follow certain principles.

(1) The principle of roughing first and then finishing

The processing order of each surface in accordance with the roughing, semi-finishing, finishing and finishing order, the purpose is to gradually improve the precision and surface quality of the parts processing surface. If all the surfaces of the parts are processed by CNC machine tools, the process is generally arranged in accordance with the order of roughing, semi-finishing and finishing, i.e. rough machining is completed before semi-finishing and finishing. Rough machining can quickly remove most of the machining allowance, and then finish machining each surface in turn, so as to improve production efficiency, but also to ensure the machining accuracy of the parts and surface roughness. The method is suitable for machining surfaces with high positional accuracy requirements. This is not absolute, such as for some size accuracy requirements of the processing surface, taking into account the stiffness of the part, deformation and size accuracy requirements, can also consider these processing surface respectively according to roughing, semi-finishing, finishing the order to complete. For high accuracy requirements of the processing surface, in roughing, finishing processes, the parts are best set aside for a period of time, so that the roughing of the parts after the surface stress is completely released, reduce the degree of stress deformation of the surface of the parts, which is conducive to improving the machining accuracy of the parts.

(2) reference surface first processing principle

Processing at the beginning, always used as a finishing benchmark surface processing out, because the positioning of the benchmark surface precision, clamping error is small, so any parts of the processing, always first positioning benchmark surface for roughing and semi-finishing, if necessary, but also finishing, for example, shaft parts are always positioning benchmark surface for roughing and semi-finishing, and then finish machining. For example, shaft parts are always machined with a central bore, then the bore system and other surfaces are machined with the central bore face and locating hole as the finishing datum. If there is more than one fine reference surface, it should be in accordance with the order of reference conversion and gradually improve the processing accuracy of the principle to arrange the processing of the reference surface.

(3) First face and then hole principle

For the box class, bracket class, the body class and other parts, plane contour size is larger, with plane positioning is more stable and reliable, so should be processed first plane, after processing holes. In this way, not only to make the subsequent processing has a stable and reliable plane as a positioning reference surface, and in the flat surface processing holes, processing becomes easier, but also conducive to improving the hole processing accuracy. Usually, you can divide the process according to the processing parts of the parts, generally first processing simple geometry, after processing complex geometry; first processing lower precision parts, after processing higher precision parts; first processing plane, after processing holes.

(4) First inside, then outside principle

For precision sleeve, its outer circle and hole coaxiality requirements are high, generally use the principle of first hole and then outer circle, that is, the outer circle as a positioning reference for processing the hole, and then a higher accuracy hole as a positioning reference for processing the outer circle, so as to ensure that the outer circle and hole have a high coaxiality requirements, and the use of fixture structure is also very simple.

(5) the principle of reducing the number of tool changes

In CNC machining, should be as far as possible in the order of the tool into the processing position to arrange the processing sequence.

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